You are hopeless at noticing change
In 1998 Simons and Levin performed an experiment to show just how bad people are at noticing change. A person with a map would approach someone, acting lost, and ask them how to get to somewhere. Whilst that person (an inadvertent participant in the experiment) tried to describe the route a couple of people barged through holding a door.
With well practised choreography the person who had asked directions switched places with one of the door carriers, who then pretended to be the person who had asked questions. The inadvertent participant would continue giving directions, and 50% of the time did not notice that the other person had changed.
This experiment demonstrated outside of the lab something that had been shown just in laboratory conditions before – the fact that we are all hopeless at noticing change. It seems preposterous that we would not notice something major changing from one second to the next, right in front of us. Actually, though, it makes perfect sense.
Look around you now and see just how much there is to notice. Imagine how much brain power it would take to actually memorise all of your surroundings, because of course to notice something has changed you first have to remember what it was before. It would take an unfeasible amount of brain storage to remember just one snapshot of our surroundings, let alone every second of every hour of every day. And what would be the purpose of remembering what is right in front of us? To know what is there we only have to look.
It would make no sense at all to remember everything we perceive from one second to the next. So we don’t. And so we often don’t realise when something has changed. And we don’t remember what passers-by look like. And we make hopeless eyewitnesses.